Various types of optical discs, including DVDs, are now used extensively as media to store digital copyrighted works such as audio video (AV) data and computer data thereon. Optical discs are roughly classifiable into read-only ones and recordable/rewritable ones (which will be referred to herein as “recordable discs” collectively). Recordable discs are manufactured so as to allow users to record some works thereon as far as their copyright is not infringed. Actually, however, it is quite possible that those recordable discs are used to make unauthorized copies of digital copyrighted works and circulate those illegal discs. To avoid such unfavorable situations, an optical disc drive needs to recognize the type of a given optical disc as read-only or recordable and do some appropriate processing for the purpose of copyright protection if the disc loaded is recordable. In order to discourage willful alternation of recordable discs for the purpose of erroneous recognition, the optical disc drive should recognize the type by sensing the difference in physical structure between the read-only and recordable discs.
Hereinafter, the physical structural difference between read-only and recordable discs will be described with reference to the accompanying drawings.
FIG. 1(a) is a perspective view partly in section illustrating a portion of the recording side of a read-only disc to show two adjacent recording tracks 140 on a larger scale.
In the read-only disc shown in FIG. 1(a), pits 141 and spaces 142 are arranged along spiral recording tracks 140 on the mirrored recording side. The pits 141 are embossed portions that are displaced perpendicularly to the mirrored recording side (i.e., concave portions or convex portions). On the other hand, the spaces 142 are mirrored portions of the recording tracks 140 on which no pits 141 are present. Information is stored there by arranging those pits 141 and spaces 142 in a unique pattern.
FIG. 1(b) is a perspective view partly in section illustrating a portion of the recording side of a recordable disc to show two adjacent grooves 143 on a larger scale. These grooves 143 are located on recording tracks.
In the recordable disc shown in FIG. 1(b), recording marks 144 and spaces 145 are arranged along the grooves 143. Either spiral or concentric grooves 143 are provided on the optical disc. Although not shown clearly in FIG. 1(b), a storage layer is provided on the optical disc. As the storage layer, either a phase change storage layer, of which the refractive index changes upon the exposure to a write laser pulse, or an organic dye film with a variable light absorbance is used extensively. The recording marks 144 are portions in which the phase of the storage layer has been changed locally upon the exposure to the laser pulse and are formed so as to be rewritable at any time. A portion with the recording mark 144 and a portion with the space 145 exhibit mutually different reflectances with respect to a read laser beam. Accordingly, by irradiating a groove with the read laser beam and by detecting the intensity of the reflected beam (i.e., the quantity of the reflected light), the arrangement of the recording marks 144 and spaces 145 can be sensed. In this manner, information may be written as needed as the arrangement of recording marks 144 and spaces 145 on the recordable disc.
As described above, the read-only disc and recordable disc have a physical structural difference between their recording sides. Grooves are present only on the recordable discs.
Japanese Laid-Open Publication No. 2001-28159 pays special attention to the grooves that are present only on those recordable discs and discloses a technique of recognizing the type of a given disc as recordable or read-only by determining whether there are grooves or not.
The groove detecting method disclosed in Japanese Laid-Open Publication No. 2001-28159 will be described with reference to FIG. 2. FIG. 2 shows a configuration for an apparatus for detecting grooves for an optical disc drive.
In the configuration shown in FIG. 2, a photodetector (PD) 200, which is divided into four areas A, B, C and D, receives a light beam that has been reflected from an optical disc and performs photoelectric conversion in each of these four areas A, B, C and D. As a result, four electric signals representing the intensities (or quantities) of the light that was incident on these four areas A, B, C and D are output.
The electric signals, output from the four areas A, B, C and D of the photodetector 200, are input to an adder circuit 201. The adder circuit 200 adds all of these electric signals together, thereby outputting an RF signal.
The RF signal is output from the adder circuit 201 to a sample-and-hold (SH) circuit 202. The sample-and-hold circuit 202 samples and holds the output RF signal of the adder circuit 201 to generate a peak hold signal.
The peak hold signal is output from the sample-and-hold circuit 202 to a low pass filter (LPF) 203. The low pass filter 203 removes high frequency components of the peak hold signal that has been supplied from the sample-and-hold circuit 202, thereby generating an upper envelope signal.
FIG. 3(a) shows the waveforms of an RF signal H and an upper envelope signal I to be obtained when a recordable disc is loaded. FIG. 3(b) shows the waveforms of an RF signal H and an upper envelope signal I to be obtained when a read-only disc is loaded. The waveforms of the RF signal H and upper envelope signal I shown in FIGS. 3(a) and 3(b) are obtained when the beam spot of a light beam on an optical disc crosses the recording tracks of the optical disc. The RF signal H is output from the adder circuit 201 shown in FIG. 2. The envelope signal I is output from the low pass filter 203 shown in FIG. 2. And the waveshape of the envelope signal I is the same as the shape of the upper envelope of the RF signal H.
If an optical disc drive is loaded with a recordable disc, a light beam spot, crossing the recording tracks, is diffracted by the grooves, thus changing the quantity of reflected light as shown in FIG. 3(a). Accordingly, the upper envelope of the RF signal H oscillates at a frequency corresponding to the number of grooves crossed by the light beam spot per unit time. Thus, the upper envelope signal I also oscillates at that frequency.
On the other hand, if the optical disc drive is loaded with a read-only disc, the upper envelope of the RF signal H hardly oscillates as shown in FIG. 3(b) even when the light beam spot crosses the recording tracks. This is because there are no grooves on the recording side of the optical disc. Accordingly, the envelope signal I hardly oscillates, either.
Thus, the optical disc drive disclosed in Japanese Laid-Open Publication No. 2001-28159 pays special attention to such a difference in waveform between the upper envelope signals I obtained from a recordable disc and a read-only disc due to the presence or absence of the grooves and recognizes the type of given disc by sensing this difference.
In the prior art, the photodetector recognizes the type of a given disc based on the total quantity of light received. However, depending on the shape of the light beam spot on the optical disc or the light quantity distribution on the grooves, the variation of the envelope signal may be too small to sense. In that case, it is difficult to determine whether the given optical disc has grooves or not. The shape of the light beam spot and the light quantity distribution may differ with the specific arrangement of an optical system for an optical head or the specific design of the optical disc and may also be affected by the tilt of the light beam and the shift of the focal point. Thus, according to the conventional technique, it is not possible to determine reliably enough whether the grooves are present or absent.
In order to overcome the problems described above, an object of the present invention is to provide an optical disc drive that can appropriately sense the difference in surface shape between multiple types of optical discs and thereby recognize the type of the given optical disc accurately without being affected by the difference in light beam spot shape or light quantity distribution.